Current candidate HIV-1 vaccines induce T cells that (i) possess restricted breadth; i.e. recognize only a few different epitopes; and (ii) predominantly target variable regions of HIV-1. Furthermore, during acute HIV-1 infection individuals who possess T cells targeting variable HIV-1 epitopes progress to disease faster than individuals possessing T cells targeting conserved HIV-1 epitopes. These data support the views that an effective T cell-based immunogen will contain only conserved regions of HIV-1 (the conserved immunogen approach). An alternative approach (the mosaic immunogen approach) has been to design immunogens to include multiple variants. This approach is based upon the theory that inclusion of multiple variants would enable responses to a broader range of circulating variants and could prime the immune system against common escape mutants. This proposal seeks to test the hypothesis that the conservation (as measured by conservation score) rather than the diversity of response against non-conserved epitopes correlates with effectiveness of vaccine-induced T cell responses. This hypothesis will be addressed in the following three Specific Aims: 1) To assess the effectiveness of T cells targeting conserved vs. variable HIV-1 epitopes to suppress viral replication in vitro, 2) To determine if T cells induced using conserved or mosaic immunogens will be activated in the context of natural HIV-1 infection and assess their ability to control viral replication; and 3) To determine if fragmentation of HIV immunogens will increase breadth of induced T cell response by increasing the pool of epitopes derived from defective ribosomal products (DRiPs). This proposal will utilize a novel in vitro priming assay to predict the epitope specificities that will be recognized by induced T cells upon vaccination with candidate vaccines. This assay could be used as a first screen of other candidate T cell based immunogens allowing only those candidates that show the most promise to move forward into more costly human clinical trials. The proposed studies have far reaching implications for design of T cell-based HIV vaccines. Determining whether effective anti-viral T cell responses induced by vaccination are due to (i) increasing the number of HIV-1 variants that are recognized or (ii) increasing specificity such that only conserved regions of HIV-1 are targeted (or both) will also have implications for the development of vaccines for other infectious organisms with high mutation rates e.g. Hepatitis C Virus (HCV).